1. Field of the Invention
The present invention relates to a flyback DC/DC converter for a power supply, and more particularly, to a flyback DC/DC converter in which a voltage across a rectifier diode on the secondary side can be reduced such that it is lower than the output voltage. In the flyback DC/DC converter, the ringing voltage of the rectifier diode can be removed to reduce a voltage stress remarkably. In addition, the efficiency of the flyback DC/DC converter can be enhanced because a snubber circuit is unnecessary.
2. Description of the Related Art
A flyback DC/DC converter has been proposed as being very suitable for a low-priced power converter that converts a DC input voltage into a DC output voltage. The flyback DC/DC converter is used in a power supply such as a switching mode power supply (SMPS). However, a conventional flyback DC/DC converter must be implemented using expensive and high-performance semiconductor derives because voltage stresses on a switch and an output diode are too high.
In addition, the conventional flyback DC/DC converter must be equipped with an RCD or an RC snubber because a high ringing voltage is generated when a power switch and a rectifier diode is turned off. Such a snubber causes a high signal loss, thus degrading the system efficiency.
Meanwhile, a flyback converter is an isolation type of a buck-boost converter. Except an I/O voltage conversion ratio, the principal operations are identical to those of the buck-boost converter.
FIG. 1 is a circuit diagram of a conventional flyback DC/DC converter.
Referring to FIG. 1, the conventional fly back DC/DC converter is simple in structure and includes a flyback driver unit 10, a transformer 20, a rectifying diode D1, an output capacitor Co, and an RC snubber 30. The flyback driver unit 10 controls an internal power switch M by a pulse width modulation (PWM) scheme or by a pulse frequency modulation (PFM) scheme to supply a primary voltage Vpr. The transformer 20 transforms the primary voltage Vpr received from the flyback driver unit 10 into a secondary voltage Vse depending on a turn ratio of a primary coil Lpr to a secondary coil Lse. The rectifying diode D1 rectifies the secondary voltage Vse received from the transformer 20. The output capacitor Co smoothes an output voltage of the rectifying diode D1. The RC snubber 30 removes a ringing voltage of the rectifying diode D1.
The transformer 20 includes a magnetization inductor Lm and a leakage inductor Llkg. The magnetization inductor Lm is connected in parallel to the primary coil Lpr to exhibit the magnetization of a magnetic core. The leakage inductor Llkg is located on the path of a primary current ipr due to the leakage of the magnetic flux of the transformer 20.
When the power switch M using a metal oxide silicon field effect transistor (MOSFET) is turned on, a high ringing voltage is generated at the rectifying diode D1. That is, when the rectifying diode D1 is turned off by the turn on of the power switch M, the leakage inductor Llkg of transformer 20 resonates with a junction capacitor (Cj) of the rectifying diode D1, thereby inducing the high ringing voltage. The RC snubber 30 is used to reduce such a surge-type ringing voltage. The RC snubber 30 may include a resistor R1 and a capacitor C1.
The transformer 20 is automatically reset by an output voltage. Therefore, the conventional flyback DC/DC converter needs no additional reset circuit and thus is very suitable for a low-priced power source.
An operation of the above conventional flyback DC/DC converter will now be described with reference to FIGS. 1 and 2.
FIG. 2 is a waveform diagram of the main signals of the convention flyback DC/DC converter illustrated in FIG. 1.
Referring to FIGS. 1 and 2, a switching signal SW alternates between a high level and a low level in accordance with a PWM scheme or a PFM scheme. Thus, a state SM of the power switch M in the flyback driver unit 10 alternates between an on state and an off state. The switching operation of the power switch M causes the primary current ipr to be input into the primary coil Lpr of the transformer 20. Accordingly, a secondary current ise flows through the secondary coil Lse of the transformer 20.
A drain-source voltage Vds of the power switch M using a MOSFET changes into a low level when the power switch M is turned on. On the other hand, the drain-source voltage Vds of the power switch M changes into a high level when the power switch M is turned off. In addition, a voltage Vd1 across the rectifying diode D1 changes into a low level when the drain-source voltage Vds changes into a high level. On the other hand, the voltage Vd1 changes into a high level when the drain-source voltage Vds changes into a low level.
Referring to FIG. 2, when the power switch M is turned off, a current iLM and a current NiLM flow through the magnetization inductor Lm when viewed respectively from the primary and secondary sides of the transformer 20. The current iLM and the current NiLM serve as magnetization currents for the magnetic core.
However, a voltage stress on the rectifying diode D1 of the flyback driver unit 10 is so high as “Vin/N+Vo”. Moreover, the ringing voltage generated due to the resonance of the leakage inductor Llkg and the junction capacitor Cj is added to the voltage stress on the rectifying diode D1. Therefore, a serious ringing voltage is generated at the both terminals of the rectifying diode D1. An additional RC snubber is further provided to absorb the serious ringing voltage. Even in this case, a ringing voltage still remains and a power loss occurs due to the RC snubber.
When a voltage across the rectifying diode increases due to the ringing voltage, the rectifying diode must have a higher breakdown voltage. However, the rectifying diode becomes more expensive with an increase in its breakdown voltage.
As described above, because a magnetization current of the transformer 20 has an offset of a load current, the availability of the transformer 20 decreases in the conventional flyback DC/DC converter. In addition, when the conventional flyback DC/DC converter has a high operating frequency and a large load capacity, it must be equipped with a separate auxiliary circuit for soft switching. Moreover, the conventional flyback DC/DC converter has a large-ripple output voltage due to its discontinuous output current.